On Wed, Apr 26, 2017 at 12:55 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Wed, Apr 26, 2017 at 11:40 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>
>> Just to see that I understand you correctly.
>>
>> I am now working on storing the following:
>>
>> /*
>> * The tuple origin.{fh,layer,uuid} is a universal unique identifier
>> * for a copy up origin, where:
>> * origin.fh - exported file handle of the lower file
>> * origin.root - exported file handle of the lower layer root
>> * origin.uuid - uuid of the lower filesystem
>
> I wouldn't even store origin.root.
>
>> *
>> * origin.{fh,root} are stored in format of a variable length binary blob
>> * with struct ovl_fh header (total blob size up to 20 bytes).
>> * uuid is stored in raw format (16 bytes) as published by sb->s_uuid.
>> */
>>
>> I intend to implement lookup as follows:
>> - compare(origin.uuid, same_lower_sb->s_uuid)
>> # layer root dentries cannot be DCACHE_DISCONNECTED, so
>> # exportfs_decode_fh ignores mnt arg and returns the cached dentry
>> - root = exportfs_decode_fh(lowerstack[0].mnt, origin.root)
>> - find layer where lowerstack[layer].dentry == root
>> - this = exportfs_decode_fh(lowerstack[layer].mnt, origin.fh)
>>
>> is_subdir() is NOT needed for decoding the layer root
>> is_subdir() is optional for decoding the lower file, because
>> it is not needed to identify the layer
>
> Hmm, we can just force exportfs_decode_fh() to return a connected
> dentry (return false from *acceptable() if the dentry is disconnected)
> before going on to iterate the layers to see which one contains it.
>
Hmm, this might work, but to quote from exportfs_decode_fh():
"It's not a directory. Life is a little more complicated."
IIUC, 'connected' means 'connected to sb root', and not
'connected to mnt root', so in the optimal case where
all lower dentries are cached, exportfs_decode_fh() will return
a connected dentry for every fh we give it regardless of the
mnt argument, so we will have to use is_subdir() to find the
right layer, which brings us back to O(numlower*depth)
With the extra cost of storing the deducible information origin.root,
we will have less complex and more efficient lookup code.
Let me try and implement it and see if I am right.
We can always discard origin.root from v4 if it turns
out to be unhelpful.
Amir.
